This invention relates to a carburetor of an internal combustion engine, and more particularly relates to a variable venturi type carburetor having an electronic control fuel injection valve.
A variable venturi type carburetor having a fuel injection valve comprises a suction piston movably mounted on a venturi portion of an intake passage, so as to vary the cross-sectional area of the venturi in accordance with the change in the amount of air being fed to the engine combustion chambers. As is well-known, the cross-sectional area of the venturi portion is controlled so that the velocity of air flowing in the venturi, that is, the vacuum level in the venturi, is always maintained at a constant value. Fuel is injected into the venturi portion from the fuel injection valve provided in the vicinity of the venturi portion, and atomized so as to be mixed with sucked air. The fuel injection valve is usually controlled electronically in accordance with the running conditions of the engine, such as, the amount of sucked air, the rate of engine revolution speed, the temperature of sucked air, the temperature of engine cooling water and so on; and/or in accordance with the running conditions of the engine measured by the signals from an O.sub.2 sensor provided in an exhaust passage, so that the necessary amount of fuel is supplied wherein the actual air fuel-ratio of the mixture is in the vicinity of the so called stoichiometric air fuel-ratio.
However, in a variable venturi type carburetor having an electronic control fuel injection valve, because the fuel injection valve is actuated and electronically controlled with the signal pulse and the pulse interval and/or the amount of fuel injection in each pulse is electronically controlled in accordance with the running conditions of the engine, fuel injection takes place periodically. In a conventional variable venturi carburetor of this type, therefore, the fuel, the particles of which are relatively large, is injected from the fuel injection valve into the venturi portion. The fuel particles then beat against a throttle valve provided at the downstream area of the venturi portion and pass through a gap between the throttle valve and an inner bore wall of the carburetor. When these particles pass through the gap, the fuel is a rich mixture which is supplied into engine combustion chambers. Subsequently, until the next fuel particles are injected and supplied through said gap, the fuel is a lean mixture which is supplied into the engine combustion chambers. Such an alternate fuel supply of the rich and lean mixtures results in, especially during the low-speed running operation of the engine, the change of engine torque, the instability of idling operation and/or surging.
When the diameter of the fuel particles is so large that the fuel is not fully mixed with sucked air and comes into collision with the throttle valve and if the position of the collision is away from the center portion of the throttle valve, there appear areas of both lean and rich mixtures. Therefore, only the rich mixture is supplied into a certain cylinder or cylinders of the multi-cylinder engine. As a result, an effective engine output power cannot be obtained.